I like the official Bittorrent. Doesn't use much resource wise (unlike azureus). Speed depends on peers. Sometimes I get 25 KB/s, sometimes I max my pipe at 350 KB/s. One important thing to do, no matter what app you use, is to limit your upload to 80% of your Up Bandwidth. When up is saturated, your down will suffer.

find out what your ip address is for the comp you have the BT client on. Log onto your router just by typing 192.168.2.1 into your web broswer (its almost always 192.168.2.1). Look for "Port forwarding" or "persistent port forwarding" in your router options (usually appears under security). Then you want to enable TCP and UDP port forwarding to your comp's ip address. To find out what port to forward take a look at the options for BT client. Azureus uses port 6881 by default